LTE Will Use Unlicensed Spectrum, But How?

Cellular carriers, who now are wholly dependent upon expensive and crowded licensed spectrum, want to set up shop in unlicensed areas. Nobody is contesting their right to do so. Quite a fight is brewing, however, over precisely how this will be done.

The reason is simple: Unlicensed spectrum is open to any and all comers, so participants must take special care to transmit only when nobody else is active.

“Wi-Fi grew up this way because in the unlicensed spectrum in general there is a diverse set of uncoordinated users and you just [have to] take this approach. LTE doesn’t. It was built for licensed spectrum in which one carrier is controlling all the spectrum. LTE assumes it has all the spectrum and goes ahead.”

Conversely, LTE evolved in a more rarified environment in which telecom companies pay for the spectrum. It’s theirs, and there is no need to play nice. Among the approaches’ main proponents are Alcatel-Lucent, Ericsson, Qualcomm Technologies and Samsung, the companies that co-founded The LTE-U Forum.

Different Approaches

LTE-U and LTE-LAA are fundamentally different approaches, though there could be convergence down the road. Wi-Fi companies’ concern is that the LTE-U approach essentially cedes control of the unlicensed spectrum to carriers. That isn’t a wise move, in their view. The fear is that the LTE-U side will pay lip service to issues of fairness, but what happens in the field will be different.

“The fundamental issue is that LTE and Wi-Fi don’t mix well together and don’t coexist well together,” Alderfer said. “When they share the same spectrum, LTE wins and Wi-Fi loses.”

Since LTE always had guaranteed access, several capabilities were never developed that are necessary when operating in the unlicensed world, according to Clint Brown, a product marketing director at Broadcom. These are the abilities to listen for traffic before transmitting, to back off if there is a collision in which two stations tranmsit simultaneously and to hear -- and thus avoid interfering with – low-level signals.

That isn’t what the other side says, of course. The LTE-U side maintains that their testing has shown that its approach results in LTE participation that is no more disruptive than a typical Wi-Fi access point (AP).

“LTE-U scans two parts of 5GHz band,” he said. “[In those two areas], it looks for vacant spectrum. If it finds it, it goes on. If not, it looks for the least occupied. It goes on that channel for a maximum on time of 50 milliseconds. The amount of time it stays on is in proportion to the number of Wi-Fi APs on the spectrum. It takes turns with those APs and goes on and off in periods of time that are in proportion to the Wi-Fi APs using the spectrum. When all the Wi-Fis have their turn, the same process is repeated.”

Significant disagreements exist between the two sides behind the pure technology. For instance, the LAA side contends that the LTE-U proponents are not fully explaining, at the highest level of technical detail, how its system works. They contend that some steps will be done in ways chosen by the cellular company and that these approaches may not be fully vetted and could create problems. Brenner insists, however, that LTE-U developers have been open and that there are no problems. “Qualcomm engineers have made extensive materials public that bear out that LTE-U causes no harm to Wi-Fi,” he said.

Compromise may be possible. Rick Svennson, the vice president of Sales and Marketing for Samsung Networks, doesn’t see it as an either/or situation. He suggested that Samsung would start out using LTE-U and evolve to LTE-LAA when it is through the standards bodies and if it is necessary. “There is no boxing match,” he said. “We believe we might move from LTE-U to LTE-LAA, which is moving at a slower pace.”

The final arbiter of how LTE accesses unlicensed spectrum will be the Federal Communications Commission (FCC). Until it does so, there are concerns: “We’re not against innovation and growth in unlicensed spectrum,” Brown said. “At the same time, we are all very concerned about a technology that can come in and have attributes that are not consistent with the standards set around access and politeness and things like that.”

Carl Weinschenk covers telecom for IT Business Edge. He writes about wireless technology, disaster recovery/business continuity, cellular services, the Internet of Things, machine-to-machine communications and other emerging technologies and platforms. He also covers net neutrality and related regulatory issues. Weinschenk has written about the phone companies, cable operators and related companies for decades and is senior editor of Broadband Technology Report. He can be reached at cweinsch@optonline.net and via twitter at @DailyMusicBrk.

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